1. Field of the Invention
The present invention relates to a wireless receiver and its demodulating method which utilizes, for example, a wireless local area network (LAN), and more specifically, relates to a technique which reduces power consumption at a demodulating circuit used for a processing device of a digital signal.
2. Description of the Related Art
The wireless receiver is driven by a battery for giving weight to its portability. Therefore, especially, in the processing device of the digital signal, the reduction in the power consumption in the demodulating circuit is desired.
A concrete configuration of a conventional technique is disclosed by Jpn. Pat. Appln. KOKAI Publication No. 2002-051016 is disclosed so as to satisfy this desire. The wireless receiver described in the aforementioned patent document enables changing an arithmetical bit length of digital signal processing and demodulates a digitized receiving signal by an instructed arithmetical bit length. The wireless receiver estimates a communication line situation from the demodulation result, and obtains the shortest arithmetical bit length satisfying a required communication quality on the basis of the estimation result to instruct the bit length to demodulating processing.
By the way, the configuration disclosed by the foregoing patent document measures a carrier-to-noise ratio (CNR) from the preamble of the receiving signal, dramatically changes the arithmetical bit length of the digital signal processing by using a branch table of a CNR versus arithmetical bit length using the conditions satisfying the required quality created from a pre-simulation (additive white Gaussian noise [AWGN]), and then obtains the shortest arithmetical bit length satisfying the required communication quality. FIG. 7 illustrates an applying range of the shortest bit length satisfying a CNE versus bit error ratio (BER) characteristic (BER=10−3) under an AWGN environment at each bit length. FIG. 8 illustrates a CNR versus bit-length branch table created on the basis of the CNR versus BER characteristic illustrated in FIG. 7.
However, the aforementioned method presumes that a propagation environment is in an ideal state, and it does not take a variation in an actual propagation environment. Therefore, especially, the environment having been causing multipath fading poses a problem that an error operation is caused because the relation between the CNE and the arithmetical bit length possible to satisfy an actual required quality does not coincide with the pre-created branch table and a bit error characteristic is degraded.